A gas turbine is a turbine driven by flowing combustion gases from a combustion chamber, which is the proper source of power. Accordingly, a gas turbine is the same thing as a jet engine where the gases, instead of flowing out into a jet, are allowed to drive a turbine rotatably connected to, for instance, an electric generator. Therefore, a usual field of application for the gas turbine is electric power generation.
Gas turbines have very high power in relation to their size and weight and are of a relatively simple construction compared to piston engines. Furthermore, gas turbines have lower emission upon combustion and have fewer movable parts than piston engines. Smaller gas turbines, so-called micro turbines, usually have low efficiency, while gas turbines of the size 1 MW and above achieve higher efficiency, however never comparable to the one of piston engines. At a power above 1 MW, however, the gas turbine soon becomes a much cheaper alternative, such that it, in spite of the difference in efficiency, may surpass the piston engine.
The work process in a gas turbine, where the main components are a compressor, a combustion chamber and a turbine, is described by the Brayton cycle, which is a thermodynamic cycle. Air from the surroundings is sucked into one or more compressor steps, where it is compressed. The compressed air is then fed to the combustion chamber wherein it is combusted together with supplied fuel. The hot flue gas formed by combustion is then allowed to expand through one or more turbine steps where it, in doing so, dissipates its energy. The turbine step is usually rotatably connected to the compressor step to drive the same, but the compressor step may also be driven by a separate motor or another source of energy.
In the ideal case, the compression and the expansion are isentropic processes, i.e., the entropy of the system remains constant. However, because of energy losses through the compressor and the turbine of a real gas turbine, these processes are adiabatic. Compression and expansion take place so fast that, in principle, there is no heat transfer to or from the working medium. When the pressure increases by compression, adiabatic heating takes place and the working medium, in this case the entrained ambient air, is heated. Adiabatic cooling occurs when the pressure decreases by expansion in the turbine. Parts of said dissipated heat can be extracted in the form of work to drive the turbine. By virtue of higher efficiency, a greater part of the heat energy can be extracted.
A micro turbine is, as the name states, a small high-speed gas turbine, which has lower pressure and temperature in the combustion chamber, compared with a greater gas turbine. A micro turbine typically has a developed power in the range of 50-500 kW and a size of about 3 m3 with a total weight of 2 t or more.
As fuel to drive a gas turbine, usually different fossil fuels are used, such as natural gas (methane, propane), petrol, diesel, fuel oil, liquefied petroleum gas, and digested sludge, or non-fossil fuel such as biogas, biodiesel, and E85 (a mixture of approx. 85% ethanol and approx. 15% petrol). In practice, natural gas is predominantly used, which has the lowest emission values. Also biomass fuels in solid form, such as wood pellets, reed, straw or grass have been targeted as candidates, but are associated with the disadvantage that the flue gas and the combustion products from the combustion chamber may cause fouling and contamination of the turbine blades when passing the turbine step, which may increase the energy losses and thereby lower the efficiency of the gas turbine. The turbine blades also risk being exposed to chemical attack in the form of corrosion, depending on which fuel is used.
Therefore, in gas turbines heated by solid biomass fuel, it is desirable to use a working medium, also called expansion gas or turbine air, which is separated from the flue gas. In a so-called indirectly heated work cycle, heat is transferred from the flue gas to the working medium by means of a heat exchanger, said media being held physically separated.
WO 03/093665 discloses an indirectly heated micro turbine that utilises the Brayton cycle in a gas turbine having separate cycles for flue gas and working medium. The combustion chamber is placed at a distance from the heat exchanger, the flue gas being fed from the combustion chamber to the heat exchanger wherein the heat thereof is transferred to the compressed working medium from the compressor.
Other examples of indirectly heated (micro) gas turbines are given by GB 2 402 172, FR 2 913 724 and US 2008/0041057.
WO 02/39045 discloses a directly heated gas turbine system, which has a ring-shaped recuperator for the transfer of heat between turbine exhaust fume and combustion air, including a compressor, a combustion chamber, and a turbine arranged within the internal diameter of the recuperator.
A disadvantage of the known technique is that it often has a low efficiency because of energy losses in the gas turbine system along the path followed by the working medium and/or of the flue gas from the compressor via the combustion chamber and the heat exchanger to the turbine, which contributes to lower the efficiency.
GB 2 420 382 and WO 02/055855 disclose indirectly heated gas turbine systems that have a heat exchanger situated inside a combustion chamber. By placing the heat exchanger inside the combustion chamber, the combustion gases generated during combustion may pass around the outside of the conduit of the heat exchanger and heat the fluid passing through the conduit, while the combustion gas and the fluid are held separated. The heat exchanger is designed to be readily dismantled and removed from the combustion chamber for cleaning.
One disadvantage of this type of construction is that it requires a relatively large space, something which prevents integration into, for example, vehicles of the private car type or other systems having requirements of compactness. Another drawback is that the heat exchanger must be cleaned at regular intervals to remove byproducts from combustion, such as soot or carbonaceous deposits.
Therefore, there is a need of developing compact, indirectly heated gas turbines having high efficiency.